The present invention relates to a photoelectric conversion element and an imaging device each having a photoelectric conversion layer composed of an organic layer configured to generate electric charges according to received light, i.e., an organic photoelectric conversion layer and each converting an optical image including at least a visible light image into an electric signal, and particularly to a photoelectric conversion element and an imaging device that are excellent in barrier properties and heat resistance.
As image sensors used for digital still cameras, digital video cameras, cameras for cellular phones, cameras for endoscopes, and the like, solid-state imaging devices (so-called CCD sensors or CMOS sensors) in which pixels having photodiodes are arrayed on a semiconductor substrate such as a silicon (Si) chip and signal electric charges corresponding to photoelectrons generated by the photodiodes of the individual pixels are obtained by a CCD-type or CMOS-type readout circuit, are widely known.
In recent years, an imaging device formed of an organic material and having an organic photoelectric conversion layer configured to generate electric charges according to received light is being studied.
The solid-state imaging device having the organic photoelectric conversion layer (hereinafter also simply called “imaging device”) is made up of a pixel electrode (lower electrode) formed on a semiconductor substrate on which a signal readout circuit is formed; the organic photoelectric conversion layer formed on the pixel electrode; a transparent counter electrode (upper electrode) formed on the organic photoelectric conversion layer; a protective film formed on the counter electrode for protecting the counter electrode; a color filter; and the like.
In the imaging device, when bias voltage is applied between the pixel electrode and the counter electrode, an exciton generated in the organic photoelectric conversion layer is dissociated into an electron and a hole, and a signal corresponding to the electric charge of the electron or hole that has moved to the pixel electrode according to the bias voltage is obtained by a CCD-type or CMOS-type readout circuit.
It is known that, in an imaging device having an organic photoelectric conversion layer, a protective film is formed on the organic photoelectric conversion layer by low temperature plasma-enhanced CVD at a temperature in the range from room temperature to about 80° C. (JP 2006-245045 A).
JP 2006-245045 A discloses a photoelectric conversion film-stacked type solid-state imaging device using an organic semiconductor and having a protective layer composed of an inorganic material which is formed above a transparent counter electrode. The inorganic material constituting the protective layer comprises silicon oxide, silicon nitride or silicon oxynitride, and the protective layer is formed by plasma-enhanced CVD capable of film formation at low temperatures (normal temperatures).
It is also known that, in an imaging device having an organic photoelectric conversion layer, a buffer layer is provided between a transparent upper electrode and a protective film in order to mitigate the influence of stress from the protective film (JP 2007-250890 A).
JP 2007-250890 A discloses a photoelectric conversion device including a first electrode formed on a flat plane of a substrate; a photoelectric conversion layer formed in an upper side of the first electrode; and a second electrode formed in an upper side of the photoelectric conversion layer, the photoelectric conversion device further including a deterioration factor adsorptive and/or reactive layer which is a layer for covering the first electrode, the photoelectric conversion layer and the second electrode and which has at least one of adsorptivity of adsorbing a deterioration factor and reactivity of reacting with the deterioration factor; and a protective layer covering the deterioration factor adsorptive and/or reactive layer so as to protect the first electrode, the photoelectric conversion layer and the second electrode.
As the deterioration factor adsorptive and/or reactive layer, a layer of a metal complex Alq3 (tris(8-hydroxyquinolinato)aluminum(III) complex) formed by resistance heating vapor deposition is disclosed. It is described that, when a layer composed of silicon nitride and obtained by plasma-enhanced CVD (layer thickness: 1 μm) is used as the protective layer, the change in dark current in the case where the deterioration factor adsorptive and/or reactive layer composed of Alq3 is formed is smaller compared to the case where the same is not formed.